Project Summary Calcium oxalate kidney stone disease occurs in approximately 9 % of the U.S. population and contributes significantly to health care costs. Small reductions in urinary oxalate excretion are thought to limit stone risk. Recent studies have indicated that a lack of Oxalobacter formigenes colonization is a significant risk factor for calcium oxalate kidney stone formation. Our Preliminary data shows that healthy human subjects lacking O.formigenes can be readily colonized with live preparations of O.formigenes. This ability to colonize individuals offers a powerful means of assessing responses to varying diets before and after colonization. It is still not clear under what conditions O.formigenes modifies stone risk, and much still needs to be learned about how this organism establishes and maintains gut colonization. In the previous funding cycle we identified in a mouse model conditions that influence the growth of O.formigenes in the gut, its degradation of oxalate in the gut, its effects on the amount of oxalate excreted in urine and into the intestine, and its sustainability in the absence of dietary oxalate. In this application we will extend these findings to humans by examining individuals before and after colonization with O.formigenes. Diets controlled in their contents of oxalate, calcium and other nutrients, including a diet ultra-low in oxalate, will be used to examine the interaction between diet, O.formigenes colonization and urinary oxalate excretion. This proposal will use spinach that contains carbon 13-oxalate to accurately determine whether colonization modifies the response to an oxalate-rich meal. Continuous intravenous infusions of the stable, non-radioactive oxalate isotope, 13C2- oxalate, before and after colonization will directly determine if gastrointestinal oxalate secretion occurs and whether colonization influences such secretion. Recent evidence suggests individuals are colonized with different strains and in some cases more than one strain. However, it is not known whether the biological differences between strains influence colonization persistence and urinary oxalate excretion and this will be examined in this application. If the specific aims are successfully completed they will provide valuable information about the importance of O.formigenes colonization in influencing oxalate levels, will create insight into the factors that impact O.formigenes colonization, and will help identify new strategies for modifying the gut milieu and calcium oxalate kidney stone risk.